# How many pack-years did I smoke?

Works out a smoking history in pack-years, packs a day times years smoked, over one or more periods, and says whether it reaches the 20 pack-years in the USPSTF lung cancer screening advice.

- Page: https://www.acalculator.org/health/pack-year-calculator
- JSON spec: https://www.acalculator.org/health/pack-year-calculator.json
- Version: 742e0d6ac7b1

## Default answer

Example with the default inputs (I count Packs a day, Smoking periods [Amount a day 1, Years 20]): Your smoking history is 20 pack-years.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| unit | I count | Whether you type how much you smoked a day in packs or in cigarettes (20 to a pack). |
| periods | Smoking periods | One row for each stretch of time you smoked about the same amount: how much a day, and for how many years. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| packYears | Pack-years | The sum over all periods of packs a day × years. |
| cigarettes | Cigarettes smoked | The total number of cigarettes over all periods: pack-years × 20 × 365. |
| years | Years smoked | The years of all periods added up. |
| screening | Screening line | Whether the history reaches the 20 pack-years in the USPSTF lung cancer screening advice. |

## Method

pack-years = Σ (packs a day × years) over each period, where packs a day = cigarettes a day ÷ 20.

## Assumptions

- A pack holds 20 cigarettes.
- Each period has one steady amount a day; split your history into rows when it changed.
- Arithmetic is exact on the typed decimals; pack-years show to one decimal, rounded half up.
- Pack-years measure exposure only. They are one part of screening advice and not a diagnosis.

## Worked examples

1. unit = packs, periods = {"amount":1,"years":20} or undefined gives packYears = 20, cigarettes = 146,000. Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02; US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02.
2. unit = packs, periods = {"amount":2,"years":0.5} or undefined gives packYears = 1. Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02: 2 packs a day for half a year is 1 pack-year.
3. unit = cigs, periods = {"amount":10,"years":30} or undefined gives packYears = 15, years = 30. Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02.
4. unit = packs, periods = {"amount":1.5,"years":10} or {"amount":0.5,"years":12} gives packYears = 21, years = 22. Source: US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02.

## FAQ

### What is a pack-year?

A measure of how much a person has smoked over time. One pack-year is smoking 1 pack (20 cigarettes) a day for 1 year. It is worked out as packs smoked a day × years smoked.

### How do I calculate pack-years?

Multiply the packs smoked a day by the years smoked. 1 pack a day for 20 years is 20 pack-years; 2 packs a day for half a year is 1 pack-year.

### How do I calculate pack-years from cigarettes a day?

Divide the cigarettes a day by 20 to get packs a day, then multiply by the years. 10 cigarettes a day for 30 years is 0.5 × 30 = 15 pack-years.

### What if I smoked different amounts at different times?

Work out each period on its own and add them up. 1.5 packs a day for 10 years and then half a pack a day for 12 years is 15 + 6 = 21 pack-years. Add one row for each period.

### Why do 20 pack-years matter?

The US Preventive Services Task Force (2021) advises a yearly low-dose CT scan for lung cancer for adults aged 50 to 80 with a history of 20 pack-years or more who smoke now or quit within the past 15 years. Talk with your clinician about screening.

### Do years since quitting lower my pack-years?

No. Pack-years count what you smoked, so they do not go down after you quit. The time since quitting matters separately: USPSTF screening stops once a person has not smoked for 15 years.

## Sources

- National Cancer Institute, NCI Dictionary of Cancer Terms, pack year: calculated by multiplying the packs of cigarettes smoked per day by the years the person has smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year (retrieved 2026-10-02)
- US Preventive Services Task Force, Lung Cancer: Screening, final recommendation (March 9, 2021): annual low-dose CT for adults aged 50 to 80 years with a 20 pack-year smoking history who currently smoke or have quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening (retrieved 2026-10-02)